---
title: "Manometer between oil and water pipes"
subject: "Hydro Power"
chapter: 1
key: "semester_6/hydro_power:manometer-oil-water-pipes"
source: https://www.bctengineer.com/q/semester_6/hydro_power/manometer-oil-water-pipes
site: BCT Engineering
---

# Manometer between oil and water pipes

Hydro Power · Chapter 1: Introduction to Basic Fluid Mechanics and Hydraulics

- Asked in: 2079 Chaitra, 6 marks
- Video: [Pressure Measurement Manometers](https://www.youtube.com/watch?v=oXXGIFSmZRE) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/manometer-oil-water-pipes
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-1

## Question

Two pipes as shown in figure convey oil of specific gravity of 0.875 and water respectively. Both the liquids in the pipes are under pressure. The pipes are connected to a U-tube manometer and the hoses connecting the pipes to the tubes are filled with the corresponding liquids. Find the difference of pressure in two pipes if the level of manometric liquid having a specific gravity of 1.25 is 2.25 m higher in the right limb than the lower level of oil in the left limb of the manometer. [Figure: pipe A (oil) and pipe B (water) connected through an inverted U-tube manometer; the centre of A is 1.5 m above the lower oil level D–D, and the manometric liquid in the right limb stands 2.25 m above D–D (level E–F).]

## Short answer

At D–D: left, p_A + 0.875×9.81×1.5 = p_A + 12.876 kN/m²; right, p_B + 1.25×9.81×2.25 = p_B + 27.591 kN/m². Equate: p_A − p_B = 14.715 kN/m², pipe A pressure higher.

## Answer

Equate the pressures on the horizontal plane D–D (the lower oil level / top of the manometric liquid in the left limb), since below it the same manometric liquid is continuous.

**Given:** oil $S_1 = 0.875$, manometric liquid $S = 1.25$, water $S = 1$. Centre of A is 1.5 m above D–D. Manometric liquid in the right limb stands 2.25 m above D–D (level E–F). The centre of B is taken at level E–F, as no other height is given.

```text
   A (oil) o                    o B (water) -- E-F
           |                    |
           | 1.5 m oil          | 2.25 m of
           |                    | liquid S=1.25
   D ------+---- S = 1.25 ------+------ D
```

#### Pressure at D–D, left limb
$$
p_A + 0.875 \times 9.81 \times 1.5 = p_A + 12.876\ \text{kN/m}^2
$$

#### Pressure at D–D, right limb
$$
p_B + 1.25 \times 9.81 \times 2.25 = p_B + 27.591\ \text{kN/m}^2
$$

#### Equate
$$
\begin{aligned}
p_A + 12.876 &= p_B + 27.591 \\
p_A - p_B &= 27.591 - 12.876 = 14.715\ \text{kN/m}^2
\end{aligned}
$$

**Answer:** $p_A - p_B = 14.715$ kN/m² (≈ 14.72 kPa); pipe A (oil) is at the higher pressure.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The datum D-D is chosen at the lower oil level because:
   A. Below it the same manometric liquid is continuous, so pressures at D-D in both limbs are equal
   B. It is the free surface
   C. It is the pipe centre
   D. It has zero pressure
   Answer: A. At D-D the pressures in both limbs are equal since the manometric liquid is continuous below. (Key point: Datum choice)

2. Pressure at D-D in the left limb is:
   A. p_A + 0.875(9.81)(1.5) = p_A + 12.876
   B. p_A - 12.876
   C. p_A + 1.25(9.81)(1.5)
   D. p_A + 9.81(1.5)
   Answer: A. The oil column 1.5 m (S = 0.875) lies above D-D, adding 12.876 kN/m^2. (Key point: Left limb)

3. Pressure at D-D in the right limb is p_B plus:
   A. 1.25 x 9.81 x 2.25 = 27.591 kN/m^2
   B. 9.81 x 2.25
   C. 0.875 x 9.81 x 2.25
   D. 1.25 x 9.81 x 1.5
   Answer: A. Manometric liquid of S = 1.25 stands 2.25 m above D-D in the right limb. (Key point: Right limb)

4. The pressure difference p_A - p_B is:
   A. 14.715 kN/m^2
   B. 40.47 kN/m^2
   C. -14.715 kN/m^2
   D. 27.59 kN/m^2
   Answer: A. p_A - p_B = 27.591 - 12.876 = 14.715 kN/m^2; A is at higher pressure. (Key point: Result)

## Related in Hydro Power

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- [Differential manometer between CCl4 and oil pipes](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Amanometer-ccl4-oil-pipes) — Hydro Power, chapter 1
- [Differential manometer pressure at point B](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Amanometer-air-water-oil) — Hydro Power, chapter 1
- [Pressure difference between containers A and B](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Amanometer-inclined-containers) — Hydro Power, chapter 1
- [Conical draft tube inlet pressure and efficiency](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aconical-draft-tube-numerical) — Hydro Power, chapter 7

## Related in other subjects

- [Differential U-tube manometer between two pipes](https://www.bctengineer.com/question.md?key=semester_4%2Ffluid_mechanics_mechanical%3Adifferential-manometer-pipes) — Fluid Mechanics, chapter 2
- [Double U-tube manometer, oil and brine pipes](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Adouble-u-tube-oil-brine-pressure) — Fluid Mechanics, chapter 2
- [Oil-water interface in enlarged-end U-tube](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Au-tube-enlarged-ends-oil-water-interface) — Fluid Mechanics, chapter 2
- [Oil-water manometer with gas cylinder](https://www.bctengineer.com/question.md?key=semester_3%2Ffluid_mechanics%3Aoil-water-manometer-gas-cylinder) — Fluid Mechanics, chapter 2